[FEATURE]: Add a visible control on the side panel tab bar to collapse it back to full-width chat

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem: In the desktop app session view, once the right-side panel (Review / Files changed / Context tabs) is opened from the header toggle, there is no visible control on the panel itself to collapse it back. It's easy to accidentally open it (e.g., clicking the header icon by mistake) and end up stuck in split view without realizing that the only way back is the small header toggle on the opposite side of the window.

Proposal: Add a collapse button at the far-right edge of the side panel's tab bar (next to the "Open in" dropdown). Clicking it collapses the right panel and returns the chat to full width, exactly like a fresh session — similar to how Claude's collapsible sidebar behaves.

Implementation notes: Small change in packages/app/src/pages/session/session-side-panel.tsx — an IconButtonV2 (close icon) inside the panel tab bar that calls view().reviewPanel.close(), with tooltip + keybind hint reusing the existing command.review.toggle label. Already implemented on my fork (feat/app-review-panel-collapse), demo GIF available; happy to open a PR referencing this issue if the team agrees this fits the product direction.

(Screenshots/GIF attached below)